Modern power electronic systems are typically used to convert the electrical energy received from a power source to the form (e.g., frequency or voltage level) demanded by a load. The electronic power circuits are composed of various components, including both active semiconductor switching devices and passive components such as capacitors, inductors, and, typically, one or more transformers. Because power electronic systems handle relatively large amounts of power, energy is lost in both the active and passive components of the power system; the energy lost is dissipated in the form of heat which must be removed from the enclosure within which the power electronic components are packaged. The efficient removal of heat from the passive and active components is important to maintain the temperature in the enclosure within normal operating temperature specifications for the components to allow their efficient operation and to enhance their operating lifetime.
A type of transformer that is becoming more widely used in high output current power electronic systems is the coaxial winding transformer (CWT). The performance of coaxial winding transformers is superior to that of conventionally wound transformers in many high power, high frequency applications. The coaxial winding transformer exhibits relatively low, and well controlled, leakage inductance and has high power densities. A perspective view of a typical prior coaxial winding transformer is shown in FIG. 1A, and a cross-section through a leg of the transformer is shown in FIG. 1B. The structure of the coaxial winding transformer includes an outer conductor 11, coaxially wound inner conductor(s) 12, an interwinding space 13, which may be filled with insulating material, and, typically, a toroidal magnetic core 14 (or several cores) mounted around the outer conductor 11. When a voltage is applied to the outer winding 11 (typically a copper tube), acting as the primary, a magnetizing current will flow in, and hence a magnetizing flux is produced by, the outer winding. The resulting flux will be tangential to circular paths outside the outer winding, and all the flux produced by the outer winding links the inner winding 12 and induces a voltage proportional to the applied voltage times the turns ratio. The inverse is essentially true when the relative permeability of the core 14 is many times the permeability of the interwinding space 13.
A significant feature of the coaxial winding transformer is that substantially no leakage field is produced by the outer winding since all of the flux produced by this winding links the inner winding. Consequently, unlike conventional winding transformers, the only flux component that penetrates the core is the magnetizing flux, allowing optimal utilization of the magnetic core. The leakage inductance is a function of the interwinding space, and can be minimized by minimizing this space.
Like any electrical component, some losses will inevitably occur in a coaxial winding transformer as power is transmitted across the primary to the secondary. The lost energy is converted to heat. Where the coaxial winding transformer is carrying very high currents, the heat dissipated in the transformer can be significant and can require that provisions be made for removing this heat from the transformer. The fact that the outer transformer winding of a coaxial winding transformer is typically made of a metal tube provides some degree of natural cooling of the transformer, although substantial portions of the outer conductor are typically surrounded by the cores 14. The rate of cooling may not be sufficient, particularly if the transformer is driven at very high power levels. For example, it is a particular advantage of the coaxial winding transformer that because no leakage flux penetrates the magnetic core, the current, and hence the power level, of the transformer can be increased without requiring that the size of the transformer be increased. Nonetheless, a coaxial winding transformer of a given size driven at very high currents and high power levels will naturally run hotter than a larger coaxial winding transformer operated at the same power level, and, of course, will have a smaller outer conductor surface area from which heat can be dissipated. One prior cooling approach, illustrated in FIG. 2, is to provide cooling tubes 17 which extend through the interior of the coaxial transformer, with a coolant liquid pumped through the tubes 17 and to a heat exchanger (not shown) to draw heat away from the transformer. Although this is an effective way of cooling the transformer windings, the cost of this approach is rather high due to the need for an active closed loop liquid cooling circuit.